The invention relates to the field of impact resistant polymers, particularly polymers based on bisphenol-A-polycarbonate.
Impact resistant polymers have found a wide variety of uses as glass substitutes in both military and non-military applications. Such polymers have been used as windshields and windscreens on vehicles, canopies and windows on aircraft, face protection devices, optical storage disks, housings for appliances and business machines and ophthalmic frames and lenses. Due to the weight advantages of these polymers relative to glass, the polymers may also find use as windows for buildings.
Among the transparent polymers which have been used are polycarbonates and acrylics. While polycarbonates have greater impact strength than acrylics, they are not typically used because of limited wearability; the scratch and chemical resistance of these materials has been poor. However, an improvement in impact resistance could lead to an improvement in cost-benefit analysis and life cycle cost and make these materials cost effective competitors to acrylics.
The chemical, physical and mechanical properties which lead to desirable impact resistance for polycarbonates have not been well understood. Polycarbonates dissipate energy upon impact by way of a yielding mode of failure which offers better protection from high energy projectiles than materials which craze and/or crack. This ductile mode of failure upon impact usually results in a punched out plug or closed hole rather than spallation; this provides better ballistic protection and better residual optics.
Numerous theories have been proposed for this ductile mode of failure as opposed to the brittle failure characteristics of many other transparent polymers but no conclusive evidence, nor any hypothesis which accounts for all of the experimental data has been offered to date, and significant improvement in ballistic protection performance in transparent polymers has not been achieved.
The molecular level structure of polycarbonate has been studied by a wide variety of experimental and computational techniques in an attempt to link the mechanical behavior to chain motions occurring at the atomistic level. Single chains of glassy polycarbonate were found to show random coil behavior through light scattering, small angle X-ray scattering and small angle neutron scattering experiments. Wide angle light scattering experiments showed a single phase behavior of glassy polycarbonate with no large regions of strong intermolecular orientation correlations, while wide angle X-ray and neutron scattering analyses indicated that there were many small regions of "enhanced order" in the bulk material, consisting of no more than two or three repeat units. These regions are too small to alter the overall random coil behavior of the chain or to show fluctuations in the optical anisotropy of polycarbonate.
Various nuclear magnetic resonance (NMR) techniques have been used to isolate the motions in polycarbonate. The most obvious motions are 180.degree. flips around the C.sub.2 axis of the phenyl groups, rotation about the C.sub.3 axes of the methyl groups and oscillations about the C.sub.2 axes of the phenyl groups accompanied by 15.degree. main chain reorientation. .sup.1 H NMR line width experiments at various applied hydrostatic pressures showed that the ring flips were suppressed by increased pressure while it was proposed that main chain motions enabled the phenyl ring flips to occur by increasing volume.
Computer models of glassy polycarbonate have been subject to hardware and software limitations. A force field specifically for polycarbonate in the amorphous bulk state was developed recently by Hutnik et al., Macromolecules, 24:5956 (1991), based on the conformational characteristics of the fragment molecules diphenyl carbonate and 2,2 diphenyl propane. They performed quasi-static chain dynamics based on previously proposed static microstructures of amorphous polycarbonate and found that there was significant cooperativity between the motions of the phenyl ring, carbonate group and main chain. More significantly, they found that there was a strong influence of chain packing on the energetics of the analyzed motions and that the phenyl ring flips had far reaching effects which were especially manifest on the carbonate conformation in "soft" regions of the structure. However, the authors concluded that their computations results were not consistent with experimental reality.
An attempt has also been made to improve the properties of polycarbonate by blending with polysulfones or polyetherimides, as reported by Coleman, "Mechanical Properties of Polycarbonate, Polysulfone and Polycarbonate-Polyetherimide Blends," Final Report, NATICK/TR-91-025, April 1991. The polysulfones and polyetherimides were found to be immiscible with polycarbonate and there was no evidence that any intermolecular reactions occurred. The polysulfones and polyetherimides were found to be too stable to undergo any chain scission reaction at temperatures which are low enough to prevent the degradation of the polycarbonate.